Invasive sea squirts spotted in Scotland

26 January 2010 by Sara Coelho

If you think the blobfish - dubbed the world's ugliest animal - is unappealing, at least it attracts sympathy for being threatened with extinction. The carpet sea squirt has no such redeeming qualities. It's not just ugly, it's also an invasive species that poses a serious threat to UK marine life. Now the critter has been spotted in Scotland.

Sea squirt

The invasive carpet sea squirt (Didemnum vexillum)

Carpet sea squirts (Didemnum vexillum) come originally from Japan, but they have travelled around the globe causing havoc in ports and marinas.

The brownish, tube-shaped sea squirts live in colonies and spread fast once established on the seabed. As the colony grows, the sea squirts smother local marine life and become a serious threat to biodiversity.

Sea squirts are a real menace for all users of the marine environment if it spreads up the coast, with considerable economic impact.

- Christine Beveridge, Scottish Association for Marine Science

The sea squirt colonies are also a problem for port authorities and fish farms, as they are "especially good at growing over underwater structures such as aquaculture equipment, boat hulls and pontoons," says Dr David Donnan, policy and advice manager at the Scottish Natural Heritage.

"We have been on the lookout for this species in Scotland as it was found in Holyhead Harbour in North Wales in 2008 and more recently in the south of England," he says. "This recent finding in Largs Yacht Haven is the first sighting in Scotland."

The sponge-like leathery critters were spotted during a routine survey by Christine Beveridge, a support scientist at the Scottish Association for Marine Science.

"This is one of our target species, so when I spotted a mussel on a pontoon, covered with a fawn coloured growth, I immediately suspected the invasive sea squirt," she says.

Beveridge says that the sea squirt may have been travelled to Scotland on the hulls of leisure boats sailing from Wales or Ireland, where colonies are already established. Hitchhiking on boats is the sea squirt's preferred mode of transportation.

Now that the carpet sea squirt has been spotted in Scotland, it could become "a real menace for all users of the marine environment if it spreads up the coast, with considerable economic impact," says Beveridge.

But it's not too late to stop it spreading. Donnan and Scottish Natural Heritage advise boat owners to keep hulls clean, free of fouling and treated with anti-fouling paint. It's also important to dispose of any fouling carefully so that it doesn't go back into the water.

"People can find out more, including an identification sheet, by visiting the Non-native species website - external link from 1 February," says Donnan.

Sea squirts, known to scientists as tunicates, are primitive chordates and belong to the same phylum as people. Most species live attached to the seabed, where they filter plankton from the passing water.